**4. Factors affecting mapping and detection results**

In general, factors affecting data quality when using small underwater vehicles (e.g., DT and AUV) to carry near-seafloor micro-topographical mapping sonars fall into five categories: horizontal positioning accuracy, vertical positioning accuracy, depth accuracy, sensor time uniformity (time requirements of the sensor are uniform, the attitude sensor time is accurate to 50 ms, the transmission time is accurate to 50 ms, and the other sensors are accurate to 1 s), and sensor location uniformity (it requires precise knowledge of the coordinates of each sensor relative to the origin of the coordinates origin and installation errors). Large-scale underwater vehicles (e.g., ROV and HOV) not only have the above five features but also have other additional characteristics, including poor stability in attitude control, acoustic transducer array port and starboard installations, and wide spacing. Hence, detailed discussion on factors affecting mapping and detection results is presented using Jiaolong HOV and its BSSS as an example. By processing the BSSS detection data collected by the Jiaolong HOV, we found that factors that mostly affect mapping quality are the HOV attitude, port and starboard positions of BSSS transducer array [7].

### **4.1 Effect of HOV attitude**

*Earth Crust*

surveys.

**3.4 Human occupied vehicle**

people and limited food, water, and oxygen.

side transducer array, and subsidiary sensors).

deformation on the transducers.

mapping sonars mounted on the ROV to obtain fine topographical map of the working area. However, unlike DT and AUV, ROV is not suitable for large-area mapping

HOV is similar to a small submarine; however, a submarine is not submersible. Equipment on an HOV typically includes a manipulator, a camera system, and a special lighting system. HOV is a more versatile underwater vehicle than a submarine, which is reflected in several applications. An HOV is designed to dive to greater depths, just like a submarine. Due to enormous pressures in the deep sea, it requires a special pressure-resistant design that carries no more than two or three

According to the database of the Manned Underwater Vehicles Professional Committee of the Marine Technology Society (MTS) of the United States, there are currently 16 HOVs worldwide with a depth capacity of over 1000 m [12]. These include the Alvin (4500 m; United States), Nautile (6000 m; France), Mir I and Mir II (6000 m; Russia), Shinkai 6500 (6500 m; Japan), and Jiaolong (7000 m; China). Jiaolong is equipped with a BSSS system to obtain accurate mapping of seafloor topography and geomorphology, similar to other great-depth HOVs, as shown in **Figure 7**. The BSSS system consists of two parts: one part is installed in the manned cabin (i.e., the master controller unit), whereas the other part is installed outside the manned cabin (i.e., the electronic cabin, port-side transducer array, starboard-

The major axis of the BSSS transducer array must be parallel to the major axis of the HOV. The transducer surface normal must have an angle of 30° above the horizontal plane. In addition, the transducer array is installed between HOV stations 4 and 5 as deformation of the mounting bracket must be minimized during lifting to avoid damaging the transducer array. In this cylindrical part of the HOV, the transducer array has a better line-type after installation; the mounting bracket is independent of the load-bearing frame, thereby reducing the impact of frame

BSSS is mainly used to obtain data on micro-topography and micro-geomorphology: ultra-short baseline (USBL) and long baseline (LBL) provide navigation and positioning data, which are essential for topographical and geomorphological mapping; underwater acoustic communication devices transmit positioning data obtained by USBL on the supporting mothership at the surface to HOV, allowing the

**38**

**Figure 7.**

*Human-occupied vehicle Jiaolong.*

Because the streamline of HOV and manual control, the attitude control stability of HOV is relatively poor; therefore, HOV attitude is a main factor that effect BSSS detection and mapping. **Figure 8** shows the seafloor micro-topography around the peak of a cold spring area. **Figure 8a** uses raw data; **Figure 8b** uses data in which filtering and smoothing have been applied to the roll angle.

#### **Figure 8.**

*Seafloor micro-topography around the peak of a cold spring area: (a) map using raw data and (b) map using data in which filtering and smoothing have been applied to the roll angle.*

**Figure 9** shows the temporal change curves recorded by BSSS for (a) roll angle, (b) pitch angle, and (c) heading angle. It can be found that there is a strong correlation between map distortion in **Figure 8a** and roll angle in **Figure 9a**.

## **4.2 Effect of port and starboard positions**

Seafloor micro-topographical detection involves multiple sensors, including LBL, USBL, DVL, fiber optic compass (FOC), and BSSS arrays. The distance of the two

**Figure 9.** *Temporal change curves recorded by BSSS for (a) roll angle, (b) pitch angle, and (c) heading angle.*

#### **Figure 10.**

*Depth-sounding results for a single ping measurement along sloped terrain in a cold spring area. The gray dotted line indicates data without correction for the installation spacing between the port and starboard transducer arrays, whereas the black dotted line indicates data with correction for the installation spacing.*

**41**

**5.1 Data pre-processing**

**Figure 11.**

1.Navigation data processing

2.Rewriting navigation data

original navigation data.

*Advanced Mapping of the Seafloor Using Sea Vehicle Mounted Sounding Technologies*

BSSS array is about 2.46 m. As BSSS detect the bottom by the two BSSS array separately, so when the bottom has a large slope, the bottom detection results are different. **Figure 10** shows depth results from a single ping measurement of sloped terrain in a cold spring area, with and without correction for the installation spacing. The detection results of two BSSS arrays are different. If there is no compensation, the

*Seafloor micro-topographical maps of sloped terrain in a cold spring area: (a) without compensation of the array spacing (b) with compensation of the array spacing [10] (permissions obtained to reprint).*

**Figure 11a** shows that when the spacing of two BSSS arrays is not taken into account, the quality of the mapping result is poor. After compensation of the spacing, the transition of the mapping result is relatively smooth, and the mapping

The raw data, including BSSS mapping data, HOV attitude data, acoustic positioning data, sound velocity data, are processed in two steps (pre-processing and post-processing) to obtain topographical and geomorphology map. Data pre-processing mostly consists of five steps: navigation data processing, rewriting navigation data, coarse error elimination, angular deviation correction, and port/ starboard position correction. Data post-processing mostly consists of 12 steps, namely installation angle correction, sound velocity correction, attitude data filtering, bathymetric data filtering, fine error elimination, field correction, side-scan data filtering, bottom tracking, angle variation gain, equalization gain, data export, and mapping. **Figure 12** shows the flow diagram of data processing and mapping.

Post-process navigation and acoustic positioning data, such as FOC, DVL,

Rewrite high-precision navigation data to the BSSS raw data file to replace

USBL, and LBL, to obtain high-precision navigation data.

quality of seafloor topographical map will be poor.

quality is improved, as shown in **Figure 11b**.

**5. Data processing and mapping method**

*DOI: http://dx.doi.org/10.5772/intechopen.83448*

*Advanced Mapping of the Seafloor Using Sea Vehicle Mounted Sounding Technologies DOI: http://dx.doi.org/10.5772/intechopen.83448*

**Figure 11.**

*Earth Crust*

**Figure 9** shows the temporal change curves recorded by BSSS for (a) roll angle, (b) pitch angle, and (c) heading angle. It can be found that there is a strong correla-

Seafloor micro-topographical detection involves multiple sensors, including LBL, USBL, DVL, fiber optic compass (FOC), and BSSS arrays. The distance of the two

*Depth-sounding results for a single ping measurement along sloped terrain in a cold spring area. The gray dotted line indicates data without correction for the installation spacing between the port and starboard transducer arrays, whereas the black dotted line indicates data with correction for the installation spacing.*

*Temporal change curves recorded by BSSS for (a) roll angle, (b) pitch angle, and (c) heading angle.*

tion between map distortion in **Figure 8a** and roll angle in **Figure 9a**.

**4.2 Effect of port and starboard positions**

**40**

**Figure 10.**

**Figure 9.**

*Seafloor micro-topographical maps of sloped terrain in a cold spring area: (a) without compensation of the array spacing (b) with compensation of the array spacing [10] (permissions obtained to reprint).*

BSSS array is about 2.46 m. As BSSS detect the bottom by the two BSSS array separately, so when the bottom has a large slope, the bottom detection results are different.

**Figure 10** shows depth results from a single ping measurement of sloped terrain in a cold spring area, with and without correction for the installation spacing. The detection results of two BSSS arrays are different. If there is no compensation, the quality of seafloor topographical map will be poor.

**Figure 11a** shows that when the spacing of two BSSS arrays is not taken into account, the quality of the mapping result is poor. After compensation of the spacing, the transition of the mapping result is relatively smooth, and the mapping quality is improved, as shown in **Figure 11b**.
